As is the case for, like, 90% of 'religious wars'.
Religion is more often a justification dressing a conflict, although there are cases where religion is genuinely a contributing or major cause of a particular war.
This is true of most wars where many other causes are brought up in a conflict sometimes they are true eg German atrocities in ww2 and sometimes they are made up eg German atrocities ww1.
In Islam, they are also present - for example, the fall of Iblis and Adam.
Unlike say the Catholic church, there is no absolute power in most churches so one cannot say with other churches 100%. However I can say that few Mulisms accept the concept of inherited sin. In fact, many non-Catholic Christians do not either although most of these do accept the concept of ancestral sin.
Not even with a POD 300 AD. IIRC there were about 10 per cent Christians in the Roman Empire at the time of Constantine. If Constantine had lost against one of his rivals, Christianity might very well have remained the religion of a minority.
Unlikely as Christianity was growing fast.
Although I think these figures should be taken with a grain of salt, what it does show is that Christians are a rapidly growing movement in Rome.
40AD.....1,000 Christians.....0.0017% of Rome's population
50 ......1,400.....................0.0023%
100 .....7,530....................0.0126%
150 .....40,496...................0.07%
200 .....217,796..................0.36%
250 .....1,171,356................1.9%
300 .....6,299,832...............10.5%
350 .....33,882,008..............56.5%
(From Stark's 'The Rise of Christianity: How the Obscure, Marginal Jesus Movement Became the Dominant Religious Force in the Western World in a Few Centuries')
As you can see they are increasing their numbers by about 500% every 50 years. I am not so sure they are going to be a minority if these growth rates continued from 300 CE with or without Constantine. Even if these growth rates slow down it will probably be the biggest religion in the Roman Empire.
Putting in here of what these figures would show without Constantine
Using an S-curve
https://stats.areppim.com/calc/calc_scurve.php
the 200 and 300 figure and a Roman Empire of 60 million and then going to 400 CE
At 350 CE without Constantine, they would be 22,907,436, they would hit Constantine 350 CE figure about 20 years later at 370 CE
I get them being the majority by 366CE with 30,983,043,
By 400 CE they are 46,418,906 well and truly the majority. Once they hit these figures the Emperors will adopt Christianity.
Probably Constantine speeded up the process by 20 years.
For Christianity not to take over would require a major competitor to suddenly appear and an effective Roman persecution of Christianity, I cannot see Rome after doing 300 doing either.